Pressure regulating valves that use a mechanical spring to urge a needle into a "seated" (i.e., closed) position are well-known in the prior art. With such valves, the needle remains in the "seated" position until the flow-line pressure exceeds a predetermined value. When the flow-line pressure exceeds the predetermined value, the needle is lifted from the valve seat, thereby opening the valve to the flow of fluid therethrough.
With pressure regulating valves commercially available at the present time, there are many "dead volumes" within the valve structure wherein flow-line fluid can become trapped. In applications such as high-pressure liquid chromatography, the existence of such trapped volumes within a pressure regulator has been found to have unacceptably deleterious consequences. Such "dead volumes", being off the direct fluid flow path through the valve structure, cannot be continuously flushed by the flowing fluid, and consequently permit the accumulation of flow-line material within the valve structure.